The present invention relates to a malfunction diagnosis apparatus for an internal combustion engine which diagnoses, for example, the deterioration of a catalyst. One example of such a device purifies the exhaust gas of the internal combustion engine, using, a catalytic converter and an air-fuel ratio feedback control unit.
According to one such system, a catalytic converter is installed in an exhaust manifold in order to remove HC, NOx, and CO contained in the exhaust gas, and an air-fuel ratio feedback control unit is provided with an O.sub.2 sensor (oxygen sensor) which is installed at the upstream side of the catalytic converter and detects an air-fuel ratio. An amount of fuel injection supplied to the internal combustion engine is controlled so that the air-fuel ratio will be a predetermined value (stoichiometric air-fuel ratio), because when the air-fuel ratio at the upstream side of the catalytic converter is at the stoichiometric air-fuel ratio, the catalytic converter is functioning effectively.
In an ordinary three-way catalyst system, when the performance of the catalytic converter itself degrades, the conversion efficiency of the injured component is reduced, even if the air-fuel ratio is accurately controlled by the aforementioned air-fuel ratio feedback control unit. Accordingly, it is desirable to determine the state of deterioration of the catalytic converter, and to warn about this when deterioration has occurred.
One apparatus for making that determination is the "Catalyst Deterioration Determination Device for Internal Combustion Engine" disclosed in U.S. Pat. No. 5,088,281, incorporated herein by reference. In this apparatus, oxygen sensors (in this case, binary sensors) are arranged at the upstream and downstream sides of the catalytic converter in order to measure the time between the instant when the output value of the upstream oxygen sensor has inverted and the instant when the output value of the downstream sensor inverts. The state of deterioration of the catalyst is determined based on the size of the measured time difference. Specifically, if the time difference is small, the catalyst is determined to be in a deteriorated state. A problem with such a catalyst deterioration determination apparatus is that the time difference fluctuates according to the engine speed, regardless of the degree of deterioration of the catalytic converter. Therefore, the accuracy of the determination of the catalytic converter deterioration is low. Further, despite the fact that the catalytic converter has deteriorated, an erroneous determination may be made that the catalytic converter is not in a deteriorated state. Further still, when the upstream sensor malfunctions, not only does the accuracy in the feedback control of the air-fuel ratio degrade, but also the accuracy in determining deterioration of the catalytic converter is degraded.
It is therefore an object of the present invention to provide a malfunction diagnosis apparatus for an internal combustion engine which is able to diagnose the deterioration of the internal combustion engine at a high accuracy, regardless of the engine speed, and which is also able to diagnose, at a high accuracy, the deterioration of a sensor (for example, an air-fuel ratio sensor on the upstream side of catalyst or the like).